/*
 * Pseudo-tty backend for pterm.
 */

#define _GNU_SOURCE

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <signal.h>
#include <assert.h>
#include <fcntl.h>
#include <termios.h>
#include <grp.h>
#include <utmp.h>
#include <pwd.h>
#include <time.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <sys/ioctl.h>
#include <errno.h>

#include "putty.h"
#include "tree234.h"

#ifndef OMIT_UTMP
#include <utmpx.h>
#endif

#ifndef FALSE
#define FALSE 0
#endif
#ifndef TRUE
#define TRUE 1
#endif

/* updwtmpx() needs the name of the wtmp file.  Try to find it. */
#ifndef WTMPX_FILE
#ifdef _PATH_WTMPX
#define WTMPX_FILE _PATH_WTMPX
#else
#define WTMPX_FILE "/var/log/wtmpx"
#endif
#endif

#ifndef LASTLOG_FILE
#ifdef _PATH_LASTLOG
#define LASTLOG_FILE _PATH_LASTLOG
#else
#define LASTLOG_FILE "/var/log/lastlog"
#endif
#endif

/*
 * Set up a default for vaguely sane systems. The idea is that if
 * OMIT_UTMP is not defined, then at least one of the symbols which
 * enable particular forms of utmp processing should be, if only so
 * that a link error can warn you that you should have defined
 * OMIT_UTMP if you didn't want any. Currently HAVE_PUTUTLINE is
 * the only such symbol.
 */
#ifndef OMIT_UTMP
#if !defined HAVE_PUTUTLINE
#define HAVE_PUTUTLINE
#endif
#endif

typedef struct pty_tag *Pty;

/*
 * The pty_signal_pipe, along with the SIGCHLD handler, must be
 * process-global rather than session-specific.
 */
static int pty_signal_pipe[2] = {-1, -1}; /* obviously bogus initial val */

struct pty_tag {
  Conf *conf;
  int master_fd, slave_fd;
  void *frontend;
  char name[FILENAME_MAX];
  pid_t child_pid;
  int term_width, term_height;
  int child_dead, finished;
  int exit_code;
  bufchain output_data;
};

/*
 * We store our pty backends in a tree sorted by master fd, so that
 * when we get an uxsel notification we know which backend instance
 * is the owner of the pty that caused it.
 */
static int pty_compare_by_fd(void *av, void *bv)
{
  Pty a = (Pty)av;
  Pty b = (Pty)bv;

  if (a->master_fd < b->master_fd)
    return -1;
  else if (a->master_fd > b->master_fd)
    return +1;
  return 0;
}

static int pty_find_by_fd(void *av, void *bv)
{
  int a = *(int *)av;
  Pty b = (Pty)bv;

  if (a < b->master_fd)
    return -1;
  else if (a > b->master_fd)
    return +1;
  return 0;
}

static tree234 *ptys_by_fd = NULL;

/*
 * We also have a tree sorted by child pid, so that when we wait()
 * in response to the signal we know which backend instance is the
 * owner of the process that caused the signal.
 */
static int pty_compare_by_pid(void *av, void *bv)
{
  Pty a = (Pty)av;
  Pty b = (Pty)bv;

  if (a->child_pid < b->child_pid)
    return -1;
  else if (a->child_pid > b->child_pid)
    return +1;
  return 0;
}

static int pty_find_by_pid(void *av, void *bv)
{
  pid_t a = *(pid_t *)av;
  Pty b = (Pty)bv;

  if (a < b->child_pid)
    return -1;
  else if (a > b->child_pid)
    return +1;
  return 0;
}

static tree234 *ptys_by_pid = NULL;

/*
 * If we are using pty_pre_init(), it will need to have already
 * allocated a pty structure, which we must then return from
 * pty_init() rather than allocating a new one. Here we store that
 * structure between allocation and use.
 *
 * Note that although most of this module is entirely capable of
 * handling multiple ptys in a single process, pty_pre_init() is
 * fundamentally _dependent_ on there being at most one pty per
 * process, so the normal static-data constraints don't apply.
 *
 * Likewise, since utmp is only used via pty_pre_init, it too must
 * be single-instance, so we can declare utmp-related variables
 * here.
 */
static Pty single_pty = NULL;

#ifndef OMIT_UTMP
static pid_t pty_utmp_helper_pid = -1;
static int pty_utmp_helper_pipe = -1;
static int pty_stamped_utmp;
static struct utmpx utmp_entry;
#endif

/*
 * pty_argv is a grievous hack to allow a proper argv to be passed
 * through from the Unix command line. Again, it doesn't really
 * make sense outside a one-pty-per-process setup.
 */
char **pty_argv;

char *pty_osx_envrestore_prefix;

static void pty_close(Pty pty);
static void pty_try_write(Pty pty);

#ifndef OMIT_UTMP
static void setup_utmp(char *ttyname, char *location)
{
#ifdef HAVE_LASTLOG
  struct lastlog lastlog_entry;
  FILE *lastlog;
#endif
  struct passwd *pw;
  struct timeval tv;

  pw = getpwuid(getuid());
  memset(&utmp_entry, 0, sizeof(utmp_entry));
  utmp_entry.ut_type = USER_PROCESS;
  utmp_entry.ut_pid = getpid();
  strncpy(utmp_entry.ut_line, ttyname + 5, lenof(utmp_entry.ut_line));
  strncpy(utmp_entry.ut_id, ttyname + 8, lenof(utmp_entry.ut_id));
  strncpy(utmp_entry.ut_user, pw->pw_name, lenof(utmp_entry.ut_user));
  strncpy(utmp_entry.ut_host, location, lenof(utmp_entry.ut_host));
  /*
   * Apparently there are some architectures where (struct
   * utmpx).ut_tv is not essentially struct timeval (e.g. Linux
   * amd64). Hence the temporary.
   */
  gettimeofday(&tv, NULL);
  utmp_entry.ut_tv.tv_sec = tv.tv_sec;
  utmp_entry.ut_tv.tv_usec = tv.tv_usec;

  setutxent();
  pututxline(&utmp_entry);
  endutxent();

  updwtmpx(WTMPX_FILE, &utmp_entry);

#ifdef HAVE_LASTLOG
  memset(&lastlog_entry, 0, sizeof(lastlog_entry));
  strncpy(lastlog_entry.ll_line, ttyname + 5, lenof(lastlog_entry.ll_line));
  strncpy(lastlog_entry.ll_host, location, lenof(lastlog_entry.ll_host));
  time(&lastlog_entry.ll_time);
  if ((lastlog = fopen(LASTLOG_FILE, "r+")) != NULL) {
    fseek(lastlog, sizeof(lastlog_entry) * getuid(), SEEK_SET);
    fwrite(&lastlog_entry, 1, sizeof(lastlog_entry), lastlog);
    fclose(lastlog);
  }
#endif

  pty_stamped_utmp = 1;
}

static void cleanup_utmp(void)
{
  struct timeval tv;

  if (!pty_stamped_utmp)
    return;

  utmp_entry.ut_type = DEAD_PROCESS;
  memset(utmp_entry.ut_user, 0, lenof(utmp_entry.ut_user));
  gettimeofday(&tv, NULL);
  utmp_entry.ut_tv.tv_sec = tv.tv_sec;
  utmp_entry.ut_tv.tv_usec = tv.tv_usec;

  updwtmpx(WTMPX_FILE, &utmp_entry);

  memset(utmp_entry.ut_line, 0, lenof(utmp_entry.ut_line));
  utmp_entry.ut_tv.tv_sec = 0;
  utmp_entry.ut_tv.tv_usec = 0;

  setutxent();
  pututxline(&utmp_entry);
  endutxent();

  pty_stamped_utmp = 0; /* ensure we never double-cleanup */
}
#endif

#ifndef NO_PTY_PRE_INIT
static void sigchld_handler(int signum)
{
  if (write(pty_signal_pipe[1], "x", 1) <= 0)
    /* not much we can do about it */;
}
#endif

#ifndef OMIT_UTMP
static void fatal_sig_handler(int signum)
{
  putty_signal(signum, SIG_DFL);
  cleanup_utmp();
  raise(signum);
}
#endif

static int pty_open_slave(Pty pty)
{
  if (pty->slave_fd < 0) {
    pty->slave_fd = open(pty->name, O_RDWR);
    cloexec(pty->slave_fd);
  }

  return pty->slave_fd;
}

static void pty_open_master(Pty pty)
{
#ifdef BSD_PTYS
  const char chars1[] = "pqrstuvwxyz";
  const char chars2[] = "0123456789abcdef";
  const char *p1, *p2;
  char master_name[20];
  struct group *gp;

  for (p1 = chars1; *p1; p1++)
    for (p2 = chars2; *p2; p2++) {
      sprintf(master_name, "/dev/pty%c%c", *p1, *p2);
      pty->master_fd = open(master_name, O_RDWR);
      if (pty->master_fd >= 0) {
        if (geteuid() == 0 || access(master_name, R_OK | W_OK) == 0) {
          /*
           * We must also check at this point that we are
           * able to open the slave side of the pty. We
           * wouldn't want to allocate the wrong master,
           * get all the way down to forking, and _then_
           * find we're unable to open the slave.
           */
          strcpy(pty->name, master_name);
          pty->name[5] = 't'; /* /dev/ptyXX -> /dev/ttyXX */

          cloexec(pty->master_fd);

          if (pty_open_slave(pty) >= 0 && access(pty->name, R_OK | W_OK) == 0)
            goto got_one;
          if (pty->slave_fd > 0)
            close(pty->slave_fd);
          pty->slave_fd = -1;
        }
        close(pty->master_fd);
      }
    }

  /* If we get here, we couldn't get a tty at all. */
  fprintf(stderr, "pterm: unable to open a pseudo-terminal device\n");
  exit(1);

got_one:

  /* We need to chown/chmod the /dev/ttyXX device. */
  gp = getgrnam("tty");
  chown(pty->name, getuid(), gp ? gp->gr_gid : -1);
  chmod(pty->name, 0600);
#else

  const int flags = O_RDWR
#ifdef O_NOCTTY
                    | O_NOCTTY
#endif
      ;

#ifdef HAVE_POSIX_OPENPT
#ifdef SET_NONBLOCK_VIA_OPENPT
  /*
   * OS X, as of 10.10 at least, doesn't permit me to set O_NONBLOCK
   * on pty master fds via the usual fcntl mechanism. Fortunately,
   * it does let me work around this by adding O_NONBLOCK to the
   * posix_openpt flags parameter, which isn't a documented use of
   * the API but seems to work. So we'll do that for now.
   */
  pty->master_fd = posix_openpt(flags | O_NONBLOCK);
#else
  pty->master_fd = posix_openpt(flags);
#endif

  if (pty->master_fd < 0) {
    perror("posix_openpt");
    exit(1);
  }
#else
  pty->master_fd = open("/dev/ptmx", flags);

  if (pty->master_fd < 0) {
    perror("/dev/ptmx: open");
    exit(1);
  }
#endif

  if (grantpt(pty->master_fd) < 0) {
    perror("grantpt");
    exit(1);
  }

  if (unlockpt(pty->master_fd) < 0) {
    perror("unlockpt");
    exit(1);
  }

  cloexec(pty->master_fd);

  pty->name[FILENAME_MAX - 1] = '\0';
  strncpy(pty->name, ptsname(pty->master_fd), FILENAME_MAX - 1);
#endif

#ifndef SET_NONBLOCK_VIA_OPENPT
  nonblock(pty->master_fd);
#endif

  if (!ptys_by_fd)
    ptys_by_fd = newtree234(pty_compare_by_fd);
  add234(ptys_by_fd, pty);
}

static Pty new_pty_struct(void)
{
  Pty pty = snew(struct pty_tag);
  pty->conf = NULL;
  bufchain_init(&pty->output_data);
  return pty;
}

/*
 * Pre-initialisation. This is here to get around the fact that GTK
 * doesn't like being run in setuid/setgid programs (probably
 * sensibly). So before we initialise GTK - and therefore before we
 * even process the command line - we check to see if we're running
 * set[ug]id. If so, we open our pty master _now_, chown it as
 * necessary, and drop privileges. We can always close it again
 * later. If we're potentially going to be doing utmp as well, we
 * also fork off a utmp helper process and communicate with it by
 * means of a pipe; the utmp helper will keep privileges in order
 * to clean up utmp when we exit (i.e. when its end of our pipe
 * closes).
 */
void pty_pre_init(void)
{
#ifndef NO_PTY_PRE_INIT

  Pty pty;

#ifndef OMIT_UTMP
  pid_t pid;
  int pipefd[2];
#endif

  pty = single_pty = new_pty_struct();

  /* set the child signal handler straight away; it needs to be set
   * before we ever fork. */
  putty_signal(SIGCHLD, sigchld_handler);
  pty->master_fd = pty->slave_fd = -1;
#ifndef OMIT_UTMP
  pty_stamped_utmp = FALSE;
#endif

  if (geteuid() != getuid() || getegid() != getgid()) {
    pty_open_master(pty);

#ifndef OMIT_UTMP
    /*
     * Fork off the utmp helper.
     */
    if (pipe(pipefd) < 0) {
      perror("pterm: pipe");
      exit(1);
    }
    cloexec(pipefd[0]);
    cloexec(pipefd[1]);
    pid = fork();
    if (pid < 0) {
      perror("pterm: fork");
      exit(1);
    } else if (pid == 0) {
      char display[128], buffer[128];
      int dlen, ret;

      close(pipefd[1]);
      /*
       * Now sit here until we receive a display name from the
       * other end of the pipe, and then stamp utmp. Unstamp utmp
       * again, and exit, when the pipe closes.
       */

      dlen = 0;
      while (1) {

        ret = read(pipefd[0], buffer, lenof(buffer));
        if (ret <= 0) {
          cleanup_utmp();
          _exit(0);
        } else if (!pty_stamped_utmp) {
          if (dlen < lenof(display))
            memcpy(display + dlen, buffer, min(ret, lenof(display) - dlen));
          if (buffer[ret - 1] == '\0') {
            /*
             * Now we have a display name. NUL-terminate
             * it, and stamp utmp.
             */
            display[lenof(display) - 1] = '\0';
            /*
             * Trap as many fatal signals as we can in the
             * hope of having the best possible chance to
             * clean up utmp before termination. We are
             * unfortunately unprotected against SIGKILL,
             * but that's life.
             */
            putty_signal(SIGHUP, fatal_sig_handler);
            putty_signal(SIGINT, fatal_sig_handler);
            putty_signal(SIGQUIT, fatal_sig_handler);
            putty_signal(SIGILL, fatal_sig_handler);
            putty_signal(SIGABRT, fatal_sig_handler);
            putty_signal(SIGFPE, fatal_sig_handler);
            putty_signal(SIGPIPE, fatal_sig_handler);
            putty_signal(SIGALRM, fatal_sig_handler);
            putty_signal(SIGTERM, fatal_sig_handler);
            putty_signal(SIGSEGV, fatal_sig_handler);
            putty_signal(SIGUSR1, fatal_sig_handler);
            putty_signal(SIGUSR2, fatal_sig_handler);
#ifdef SIGBUS
            putty_signal(SIGBUS, fatal_sig_handler);
#endif
#ifdef SIGPOLL
            putty_signal(SIGPOLL, fatal_sig_handler);
#endif
#ifdef SIGPROF
            putty_signal(SIGPROF, fatal_sig_handler);
#endif
#ifdef SIGSYS
            putty_signal(SIGSYS, fatal_sig_handler);
#endif
#ifdef SIGTRAP
            putty_signal(SIGTRAP, fatal_sig_handler);
#endif
#ifdef SIGVTALRM
            putty_signal(SIGVTALRM, fatal_sig_handler);
#endif
#ifdef SIGXCPU
            putty_signal(SIGXCPU, fatal_sig_handler);
#endif
#ifdef SIGXFSZ
            putty_signal(SIGXFSZ, fatal_sig_handler);
#endif
#ifdef SIGIO
            putty_signal(SIGIO, fatal_sig_handler);
#endif
            setup_utmp(pty->name, display);
          }
        }
      }
    } else {
      close(pipefd[0]);
      pty_utmp_helper_pid = pid;
      pty_utmp_helper_pipe = pipefd[1];
    }
#endif
  }

  /* Drop privs. */
  {
#ifndef HAVE_NO_SETRESUID
    int gid = getgid(), uid = getuid();
    int setresgid(gid_t, gid_t, gid_t);
    int setresuid(uid_t, uid_t, uid_t);
    if (setresgid(gid, gid, gid) < 0) {
      perror("setresgid");
      exit(1);
    }
    if (setresuid(uid, uid, uid) < 0) {
      perror("setresuid");
      exit(1);
    }
#else
    if (setgid(getgid()) < 0) {
      perror("setgid");
      exit(1);
    }
    if (setuid(getuid()) < 0) {
      perror("setuid");
      exit(1);
    }
#endif
  }

#endif /* NO_PTY_PRE_INIT */
}

void pty_real_select_result(Pty pty, int event, int status)
{
  char buf[4096];
  int ret;
  int finished = FALSE;

  if (event < 0) {
    /*
     * We've been called because our child process did
     * something. `status' tells us what.
     */
    if ((WIFEXITED(status) || WIFSIGNALED(status))) {
      /*
       * The primary child process died. We could keep
       * the terminal open for remaining subprocesses to
       * output to, but conventional wisdom seems to feel
       * that that's the Wrong Thing for an xterm-alike,
       * so we bail out now (though we don't necessarily
       * _close_ the window, depending on the state of
       * Close On Exit). This would be easy enough to
       * change or make configurable if necessary.
       */
      pty->exit_code = status;
      pty->child_dead = TRUE;
      del234(ptys_by_pid, pty);
      finished = TRUE;
    }
  } else {
    if (event == 1) {

      ret = read(pty->master_fd, buf, sizeof(buf));

      /*
       * Clean termination condition is that either ret == 0, or ret
       * < 0 and errno == EIO. Not sure why the latter, but it seems
       * to happen. Boo.
       */
      if (ret == 0 || (ret < 0 && errno == EIO)) {
        /*
         * We assume a clean exit if the pty has closed but the
         * actual child process hasn't. The only way I can
         * imagine this happening is if it detaches itself from
         * the pty and goes daemonic - in which case the
         * expected usage model would precisely _not_ be for
         * the pterm window to hang around!
         */
        finished = TRUE;
        if (!pty->child_dead)
          pty->exit_code = 0;
      } else if (ret < 0) {
        perror("read pty master");
        exit(1);
      } else if (ret > 0) {
        from_backend(pty->frontend, 0, buf, ret);
      }
    } else if (event == 2) {
      /*
       * Attempt to send data down the pty.
       */
      pty_try_write(pty);
    }
  }

  if (finished && !pty->finished) {
    int close_on_exit;

    uxsel_del(pty->master_fd);
    pty_close(pty);
    pty->master_fd = -1;

    pty->finished = TRUE;

    /*
     * This is a slight layering-violation sort of hack: only
     * if we're not closing on exit (COE is set to Never, or to
     * Only On Clean and it wasn't a clean exit) do we output a
     * `terminated' message.
     */
    close_on_exit = conf_get_int(pty->conf, CONF_close_on_exit);
    if (close_on_exit == FORCE_OFF ||
        (close_on_exit == AUTO && pty->exit_code != 0)) {
      char message[512];
      message[0] = '\0';
      if (WIFEXITED(pty->exit_code))
        sprintf(message,
                "\r\n[pterm: process terminated with exit"
                " code %d]\r\n",
                WEXITSTATUS(pty->exit_code));
      else if (WIFSIGNALED(pty->exit_code))
#ifdef HAVE_NO_STRSIGNAL
        sprintf(message,
                "\r\n[pterm: process terminated on signal"
                " %d]\r\n",
                WTERMSIG(pty->exit_code));
#else
        sprintf(message,
                "\r\n[pterm: process terminated on signal"
                " %d (%.400s)]\r\n",
                WTERMSIG(pty->exit_code),
                strsignal(WTERMSIG(pty->exit_code)));
#endif
      from_backend(pty->frontend, 0, message, strlen(message));
    }

    notify_remote_exit(pty->frontend);
  }
}

void pty_select_result(int fd, int event)
{
  Pty pty;

  if (fd == pty_signal_pipe[0]) {
    pid_t pid;
    int status;
    char c[1];

    if (read(pty_signal_pipe[0], c, 1) <= 0)
      /* ignore error */;
    /* ignore its value; it'll be `x' */

    do {
      pid = waitpid(-1, &status, WNOHANG);

      pty = find234(ptys_by_pid, &pid, pty_find_by_pid);

      if (pty)
        pty_real_select_result(pty, -1, status);
    } while (pid > 0);
  } else {
    pty = find234(ptys_by_fd, &fd, pty_find_by_fd);

    if (pty)
      pty_real_select_result(pty, event, 0);
  }
}

static void pty_uxsel_setup(Pty pty)
{
  int rwx;

  rwx = 1; /* always want to read from pty */
  if (bufchain_size(&pty->output_data))
    rwx |= 2; /* might also want to write to it */
  uxsel_set(pty->master_fd, rwx, pty_select_result);

  /*
   * In principle this only needs calling once for all pty
   * backend instances, but it's simplest just to call it every
   * time; uxsel won't mind.
   */
  uxsel_set(pty_signal_pipe[0], 1, pty_select_result);
}

/*
 * Called to set up the pty.
 *
 * Returns an error message, or NULL on success.
 *
 * Also places the canonical host name into `realhost'. It must be
 * freed by the caller.
 */
static const char *pty_init(void *frontend,
                            void **backend_handle,
                            Conf *conf,
                            const char *host,
                            int port,
                            char **realhost,
                            int nodelay,
                            int keepalive)
{
  int slavefd;
  pid_t pid, pgrp;
#ifndef NOT_X_WINDOWS /* for Mac OS X native compilation */
  long windowid;
#endif
  Pty pty;

  if (single_pty) {
    pty = single_pty;
    assert(pty->conf == NULL);
  } else {
    pty = new_pty_struct();
    pty->master_fd = pty->slave_fd = -1;
#ifndef OMIT_UTMP
    pty_stamped_utmp = FALSE;
#endif
  }

  pty->frontend = frontend;
  *backend_handle = NULL; /* we can't sensibly use this, sadly */

  pty->conf = conf_copy(conf);
  pty->term_width = conf_get_int(conf, CONF_width);
  pty->term_height = conf_get_int(conf, CONF_height);

  if (pty->master_fd < 0)
    pty_open_master(pty);

#ifndef OMIT_UTMP
  /*
   * Stamp utmp (that is, tell the utmp helper process to do so),
   * or not.
   */
  if (pty_utmp_helper_pipe >= 0) { /* if it's < 0, we can't anyway */
    if (!conf_get_int(conf, CONF_stamp_utmp)) {
      close(pty_utmp_helper_pipe); /* just let the child process die */
      pty_utmp_helper_pipe = -1;
    } else {
      const char *location = get_x_display(pty->frontend);
      int len = strlen(location) + 1, pos = 0; /* +1 to include NUL */
      while (pos < len) {
        int ret = write(pty_utmp_helper_pipe, location + pos, len - pos);
        if (ret < 0) {
          perror("pterm: writing to utmp helper process");
          close(pty_utmp_helper_pipe); /* arrgh, just give up */
          pty_utmp_helper_pipe = -1;
          break;
        }
        pos += ret;
      }
    }
  }
#endif

#ifndef NOT_X_WINDOWS /* for Mac OS X native compilation */
  windowid = get_windowid(pty->frontend);
#endif

  /*
   * Fork and execute the command.
   */
  pid = fork();
  if (pid < 0) {
    perror("fork");
    exit(1);
  }

  if (pid == 0) {
    struct termios attrs;

    /*
     * We are the child.
     */

    if (pty_osx_envrestore_prefix) {
      int plen = strlen(pty_osx_envrestore_prefix);
      extern char **environ;
      char **ep;

    restart_osx_env_restore:
      for (ep = environ; *ep; ep++) {
        char *e = *ep;

        if (!strncmp(e, pty_osx_envrestore_prefix, plen)) {
          int unset = (e[plen] == 'u');
          char *pname = dupprintf("%.*s", (int)strcspn(e, "="), e);
          char *name = pname + plen + 1;
          char *value = e + strcspn(e, "=");
          if (*value)
            value++;
          value = dupstr(value);
          if (unset)
            unsetenv(name);
          else
            setenv(name, value, 1);
          unsetenv(pname);
          sfree(pname);
          sfree(value);
          goto restart_osx_env_restore;
        }
      }
    }

    slavefd = pty_open_slave(pty);
    if (slavefd < 0) {
      perror("slave pty: open");
      _exit(1);
    }

    close(pty->master_fd);
    noncloexec(slavefd);
    dup2(slavefd, 0);
    dup2(slavefd, 1);
    dup2(slavefd, 2);
    close(slavefd);
    setsid();
#ifdef TIOCSCTTY
    ioctl(0, TIOCSCTTY, 1);
#endif
    pgrp = getpid();
    tcsetpgrp(0, pgrp);

    /*
     * Set up configuration-dependent termios settings on the new
     * pty. Linux would have let us do this on the pty master
     * before we forked, but that fails on OS X, so we do it here
     * instead.
     */
    if (tcgetattr(0, &attrs) == 0) {
      /*
       * Set the backspace character to be whichever of ^H and
       * ^? is specified by bksp_is_delete.
       */
      attrs.c_cc[VERASE] =
          conf_get_int(conf, CONF_bksp_is_delete) ? '\177' : '\010';

      /*
       * Set the IUTF8 bit iff the character set is UTF-8.
       */
#ifdef IUTF8
      if (frontend_is_utf8(frontend))
        attrs.c_iflag |= IUTF8;
      else
        attrs.c_iflag &= ~IUTF8;
#endif

      tcsetattr(0, TCSANOW, &attrs);
    }

    setpgid(pgrp, pgrp);
    {
      int ptyfd = open(pty->name, O_WRONLY, 0);
      if (ptyfd >= 0)
        close(ptyfd);
    }
    setpgid(pgrp, pgrp);
    {
      char *term_env_var =
          dupprintf("TERM=%s", conf_get_str(conf, CONF_termtype));
      putenv(term_env_var);
      /* We mustn't free term_env_var, as putenv links it into the
       * environment in place.
       */
    }
#ifndef NOT_X_WINDOWS /* for Mac OS X native compilation */
    {
      char *windowid_env_var = dupprintf("WINDOWID=%ld", windowid);
      putenv(windowid_env_var);
      /* We mustn't free windowid_env_var, as putenv links it into the
       * environment in place.
       */
    }
    {
      /*
       * In case we were invoked with a --display argument that
       * doesn't match DISPLAY in our actual environment, we
       * should set DISPLAY for processes running inside the
       * terminal to match the display the terminal itself is
       * on.
       */
      const char *x_display = get_x_display(pty->frontend);
      char *x_display_env_var = dupprintf("DISPLAY=%s", x_display);
      putenv(x_display_env_var);
      /* As above, we don't free this. */
    }
#endif
    {
      char *key, *val;

      for (val = conf_get_str_strs(conf, CONF_environmt, NULL, &key);
           val != NULL;
           val = conf_get_str_strs(conf, CONF_environmt, key, &key)) {
        char *varval = dupcat(key, "=", val, NULL);
        putenv(varval);
        /*
         * We must not free varval, since putenv links it
         * into the environment _in place_. Weird, but
         * there we go. Memory usage will be rationalised
         * as soon as we exec anyway.
         */
      }
    }

    /*
     * SIGINT, SIGQUIT and SIGPIPE may have been set to ignored by
     * our parent, particularly by things like sh -c 'pterm &' and
     * some window or session managers. SIGPIPE was also
     * (potentially) blocked by us during startup. Reverse all
     * this for our child process.
     */
    putty_signal(SIGINT, SIG_DFL);
    putty_signal(SIGQUIT, SIG_DFL);
    putty_signal(SIGPIPE, SIG_DFL);
    block_signal(SIGPIPE, 0);
    if (pty_argv) {
      /*
       * Exec the exact argument list we were given.
       */
      execvp(pty_argv[0], pty_argv);
      /*
       * If that fails, and if we had exactly one argument, pass
       * that argument to $SHELL -c.
       *
       * This arranges that we can _either_ follow 'pterm -e'
       * with a list of argv elements to be fed directly to
       * exec, _or_ with a single argument containing a command
       * to be parsed by a shell (but, in cases of doubt, the
       * former is more reliable).
       *
       * A quick survey of other terminal emulators' -e options
       * (as of Debian squeeze) suggests that:
       *
       *  - xterm supports both modes, more or less like this
       *  - gnome-terminal will only accept a one-string shell command
       *  - Eterm, kterm and rxvt will only accept a list of
       *    argv elements (as did older versions of pterm).
       *
       * It therefore seems important to support both usage
       * modes in order to be a drop-in replacement for either
       * xterm or gnome-terminal, and hence for anyone's
       * plausible uses of the Debian-style alias
       * 'x-terminal-emulator'...
       */
      if (pty_argv[1] == NULL) {
        char *shell = getenv("SHELL");
        if (shell)
          execl(shell, shell, "-c", pty_argv[0], (void *)NULL);
      }
    } else {
      char *shell = getenv("SHELL");
      char *shellname;
      if (conf_get_int(conf, CONF_login_shell)) {
        char *p = strrchr(shell, '/');
        shellname = snewn(2 + strlen(shell), char);
        p = p ? p + 1 : shell;
        sprintf(shellname, "-%s", p);
      } else
        shellname = shell;
      execl(getenv("SHELL"), shellname, (void *)NULL);
    }

    /*
     * If we're here, exec has gone badly foom.
     */
    perror("exec");
    _exit(127);
  } else {
    pty->child_pid = pid;
    pty->child_dead = FALSE;
    pty->finished = FALSE;
    if (pty->slave_fd > 0)
      close(pty->slave_fd);
    if (!ptys_by_pid)
      ptys_by_pid = newtree234(pty_compare_by_pid);
    add234(ptys_by_pid, pty);
  }

  if (pty_signal_pipe[0] < 0) {
    if (pipe(pty_signal_pipe) < 0) {
      perror("pipe");
      exit(1);
    }
    cloexec(pty_signal_pipe[0]);
    cloexec(pty_signal_pipe[1]);
  }
  pty_uxsel_setup(pty);

  *backend_handle = pty;

  *realhost = dupstr("");

  return NULL;
}

static void pty_reconfig(void *handle, Conf *conf)
{
  Pty pty = (Pty)handle;
  /*
   * We don't have much need to reconfigure this backend, but
   * unfortunately we do need to pick up the setting of Close On
   * Exit so we know whether to give a `terminated' message.
   */
  conf_copy_into(pty->conf, conf);
}

/*
 * Stub routine (never called in pterm).
 */
static void pty_free(void *handle)
{
  Pty pty = (Pty)handle;

  /* Either of these may fail `not found'. That's fine with us. */
  del234(ptys_by_pid, pty);
  del234(ptys_by_fd, pty);

  bufchain_clear(&pty->output_data);

  conf_free(pty->conf);
  pty->conf = NULL;

  if (pty == single_pty) {
    /*
     * Leave this structure around in case we need to Restart
     * Session.
     */
  } else {
    sfree(pty);
  }
}

static void pty_try_write(Pty pty)
{
  void *data;
  int len, ret;

  assert(pty->master_fd >= 0);

  while (bufchain_size(&pty->output_data) > 0) {
    bufchain_prefix(&pty->output_data, &data, &len);
    ret = write(pty->master_fd, data, len);

    if (ret < 0 && (errno == EWOULDBLOCK)) {
      /*
       * We've sent all we can for the moment.
       */
      break;
    }
    if (ret < 0) {
      perror("write pty master");
      exit(1);
    }
    bufchain_consume(&pty->output_data, ret);
  }

  pty_uxsel_setup(pty);
}

/*
 * Called to send data down the pty.
 */
static int pty_send(void *handle, const char *buf, int len)
{
  Pty pty = (Pty)handle;

  if (pty->master_fd < 0)
    return 0; /* ignore all writes if fd closed */

  bufchain_add(&pty->output_data, buf, len);
  pty_try_write(pty);

  return bufchain_size(&pty->output_data);
}

static void pty_close(Pty pty)
{
  if (pty->master_fd >= 0) {
    close(pty->master_fd);
    pty->master_fd = -1;
  }
#ifndef OMIT_UTMP
  if (pty_utmp_helper_pipe >= 0) {
    close(pty_utmp_helper_pipe); /* this causes utmp to be cleaned up */
    pty_utmp_helper_pipe = -1;
  }
#endif
}

/*
 * Called to query the current socket sendability status.
 */
static int pty_sendbuffer(void *handle)
{
  /* Pty pty = (Pty)handle; */
  return 0;
}

/*
 * Called to set the size of the window
 */
static void pty_size(void *handle, int width, int height)
{
  Pty pty = (Pty)handle;
  struct winsize size;

  pty->term_width = width;
  pty->term_height = height;

  size.ws_row = (unsigned short)pty->term_height;
  size.ws_col = (unsigned short)pty->term_width;
  size.ws_xpixel =
      (unsigned short)pty->term_width * font_dimension(pty->frontend, 0);
  size.ws_ypixel =
      (unsigned short)pty->term_height * font_dimension(pty->frontend, 1);
  ioctl(pty->master_fd, TIOCSWINSZ, (void *)&size);
  return;
}

/*
 * Send special codes.
 */
static void pty_special(void *handle, Telnet_Special code)
{
  /* Pty pty = (Pty)handle; */
  /* Do nothing! */
  return;
}

/*
 * Return a list of the special codes that make sense in this
 * protocol.
 */
static const struct telnet_special *pty_get_specials(void *handle)
{
  /* Pty pty = (Pty)handle; */
  /*
   * Hmm. When I get round to having this actually usable, it
   * might be quite nice to have the ability to deliver a few
   * well chosen signals to the child process - SIGINT, SIGTERM,
   * SIGKILL at least.
   */
  return NULL;
}

static int pty_connected(void *handle)
{
  /* Pty pty = (Pty)handle; */
  return TRUE;
}

static int pty_sendok(void *handle)
{
  /* Pty pty = (Pty)handle; */
  return 1;
}

static void pty_unthrottle(void *handle, int backlog)
{
  /* Pty pty = (Pty)handle; */
  /* do nothing */
}

static int pty_ldisc(void *handle, int option)
{
  /* Pty pty = (Pty)handle; */
  return 0; /* neither editing nor echoing */
}

static void pty_provide_ldisc(void *handle, void *ldisc)
{
  /* Pty pty = (Pty)handle; */
  /* This is a stub. */
}

static void pty_provide_logctx(void *handle, void *logctx)
{
  /* Pty pty = (Pty)handle; */
  /* This is a stub. */
}

static int pty_exitcode(void *handle)
{
  Pty pty = (Pty)handle;
  if (!pty->finished)
    return -1; /* not dead yet */
  else
    return pty->exit_code;
}

static int pty_cfg_info(void *handle)
{
  /* Pty pty = (Pty)handle; */
  return 0;
}

Backend pty_backend = {pty_init,
                       pty_free,
                       pty_reconfig,
                       pty_send,
                       pty_sendbuffer,
                       pty_size,
                       pty_special,
                       pty_get_specials,
                       pty_connected,
                       pty_exitcode,
                       pty_sendok,
                       pty_ldisc,
                       pty_provide_ldisc,
                       pty_provide_logctx,
                       pty_unthrottle,
                       pty_cfg_info,
                       NULL /* test_for_upstream */,
                       "pty",
                       -1,
                       0};
